The present invention relates to a method for operating a combustion engine having an exhaust-gas turbocharger, in which it is possible to switch between normal operation and special operation, and at least one main fuel injection and at least one secondary fuel injection are provided in special operation.
A method for operating a combustion engine, in which a change is made possible between lean operation and rich operation in a diesel engine, is described in German Published Patent Application No. 197 50 226. The switch to lean operation or rich operation of the diesel engine occurs depending on predetermined switching criteria. Separate mappings for the operation of the diesel engine are stored for lean operation and for rich operation.
A method for operating a combustion engine having an exhaust-gas turbocharger, in which it is possible to switch between normal operation and special operation, the special operation being used to reduce turbo lag for a diesel engine with an exhaust-gas turbocharger, is described in German Published Patent Application No. 199 51 096. In response to a positive load jump, the switch-over to special operation is made in which the exhaust gas temperature and/or the exhaust gas pressure, upstream of the exhaust-gas turbocharger, is/are increased relative to normal operation. In special operation, a main fuel injection and a secondary fuel injection can be provided, enabling an essentially complete combustion of the fuel of the secondary injection quantity. During special operation, the diesel engine is operated stoichiometrically or hyperstoichiometrically, i.e., lean, if it is provided with an emissions control system.
Modern emissions control systems require a change-over of the exhaust gas composition from rich to lean. Compared to a normally lean combustion, a rich combustion is necessary from time to time, for example, in order to regenerate NOx adsorber systems.
To also be able to take measures for avoiding the so-called turbo lag during these regeneration phases without interfering with the regeneration process, it should also be possible to adjust the exhaust gas composition optionally to lean or rich, when these measures are in progress.
It is an object of the present invention to enable an accelerated build-up in charge-air pressure for a combustion engine with an exhaust-gas turbocharger while optionally adjusting a rich or lean exhaust gas composition at the same time.
According to one aspect of the present invention, a method is provided in which in reaction to a positive load jump, a switch occurs between normal operation and special operation, provided for increasing the exhaust gas enthalpy, the secondary fuel injection in the special operation being adjusted so that it burns substantially in the combustion chamber and causes an increase in exhaust gas enthalpy as well as an adjustment of a rich or lean exhaust gas composition.
The secondary injection, burned in the combustion chamber, contributes to the engine load and results in an increase in exhaust gas enthalpy. At the same time, an adjustment of the exhaust gas composition between rich and lean may be made with the aid of the secondary fuel injection. The quantity and instant of the secondary fuel injection are important adjustment parameters for engine load, increase in the exhaust gas enthalpy, and adjustment of the exhaust gas composition.
Turbochargers utilize the exhaust gas enthalpy to drive a compressor on the fresh air side. After a sudden positive load variation, the turbocharger must be accelerated. The exhaust gas enthalpy necessary may not be made available immediately with conventional operating methods, since it is limited by the smoke limit. The method of the present invention produces a higher exhaust gas enthalpy at the same smoke value and thus accelerates the charge-air pressure build-up, i.e., it reduces the turbo lag.
The above and other beneficial objects of the present invention are also achieved by providing a method in which in response to a positive load jump, a switch-over occurs between the normal operation and the special operation provided for increasing the exhaust gas enthalpy, the secondary fuel injection in the special operation being adjusted so that it burns substantially in the combustion chamber and causes an increase in the exhaust gas enthalpy and that in the special operation at least one additional offset secondary injection of fuel is provided via which a setting of a rich or lean exhaust gas composition essentially occurs.
By the provision of an additional offset secondary injection following the secondary injection burned in the combustion chamber, more flexible and precise possibilities for adjusting the engine load, exhaust gas enthalpy, and exhaust gas composition are achieved. For setting a lean or rich exhaust gas composition, the quantity and instant of the secondary injection burned in the combustion chamber, for example, which affect the engine load, need not be changed. Rather, it is sufficient to change the quantity and instant of the offset secondary injection.
In normal and special operation, depending on the operating state of the combustion engine, a pre-injection of fuel may be performed, the start of injection of which may be substantially the same in normal and in special operation.
A start of the main injection may be substantially the same in normal and in special operation.
Because the start of a pre-injection of fuel and a main injection of fuel substantially does not change between normal and special operation, a maximum pressure rise in the combustion chamber and a position of maximum pressure rise in the combustion chamber remain substantially the same between normal and special operation. Thus, a switch between normal and special operation is imperceptible and inaudible to a driver of a motor vehicle in which the combustion engine is operated in accordance with the method of the present invention.
A switching time between normal operation and special operation and a main injection quantity during the switch-over time may be adjusted as a function of an engine torque, a derivation of the engine torque, an engine speed, a derivation of the engine speed, a road speed, an engaged gear, a total injection quantity, an air mass in the intake-tract length, a derivation of the air mass, a water temperature in the cooling system, an outside air temperature, an intake manifold temperature, an exhaust gas temperature, an ambient pressure, an intake manifold pressure, and/or an exhaust gas pressure.
These measures permit a slow or fast switch-over between normal and special operation, so as to ensure that the switching procedure is almost imperceptible to a driver despite a rapid build-up in charge-air pressure.